Much progress in the electronics industry comes from circuit size reduction. This is most directly accomplished by running photolithographic processes at ever-shorter wavelengths of light. The electronics industry is currently implementing photolithographic processes employing wavelengths in the so-called “vacuum ultra-violet” (VUV). Processes using 193 nanometer (nm) light undergoing commercialization while 157 nm wavelength light is under development as a next generation candidate.
Whether a polymer or a lower molecular weight organic composition, if the material resides in the light path between the source and the target, the material needs to be transparent and photochemically stable.
For a material to be useful in VUV photolithography it is necessary but not sufficient that it exhibit high transparency, particularly at 157 nm and 193 nm; it must also exhibit high photochemical stability known in the art as radiation durability. Radiation durability is the property of a material to retain transparency upon being subject to exposure to electromagnetic radiation of a particular frequency. In many aspects of photolithography, commercial considerations require a transparent material to retain a high degree of transparency while being subject to a significant cumulative dose of radiation.
Vacuum ultraviolet radiation is of sufficiently high energy to break chemical bonds in some normally stable materials resulting in the formation of highly reactive free-radicals. It will be appreciated by one of skill in the art that the generation of a small number of free radicals can have catastrophic effects on the chemical stability of the host material by virtue of a free-radical chain reaction. The role of free-radicals in photochemical degradation of materials is well known. There are many types of free radicals, including hydroxyl radicals, oxygen radicals, and organic radicals. These free radicals are generated when sufficient energy is absorbed by a precursor molecule to cause it to dissociate non-ionically, forming species of neutral charge but sporting an unpaired electron.
Titze in Photodissoziation von H2O bei 157 nm, Max Planck Inst., Gottingen, Germany, 1984, discloses photolysis of water at 157 nm to form hydrogen and hydroxide radicals.
A. C. Fozza, J. E. Klemberg-Sapieha, and M. R. Wertheimer, Plasmas and Polymers, Vol. 4, No. 2/3, 1999, pages 183-206, discusses oxygen's undergoing photo-dissociation to activated oxygen atoms at wavelengths less than 170 nm. Also disclosed are bond breaking reactions that occur in the vacuum ultraviolet with polyethylene, polystyrene, and polymethymethacrylate. V. N. Vasilets, I. Hirata, H. Iwata, Y. Ikada, Journal of Polymer Science: Part A: Polymer Chemistry, Vol 36, 2215-2222 (1998) discusses radical formation and photooxidation when tetrafluoroethylene/hexafluoropropylene copolymer is irradiated with 147 nm light:
N. Ichinose and S. Kawanishi, Macromolecules, 1996, 29, 4155-4157 discloses the irradiation of polymers such as Teflon® PTFE, Teflon® FEP, Teflon® PFA, Tefzel®, and polyvinylidene fluoride with light at 185, 193, 248, and 254 nm. When the polymer surface was in contact with nitrogen-purged water, extensive surface reaction was detected. The surface reactivity was particularly apparent at 185, 193, and 248 nm but much less so if at all at 254 nm. Perfluorinated polymers such as Teflon® PTFE and Teflon® PFA reacted more readily than partially fluorinated polymers such as Tefzel® and polyvinylidene fluoride. No significant photochemistry was observed in the absence of water. Saturation of the water with oxygen also completely inhibited the surface chemistry. It is further taught that water starts to absorb around 190 to 200 nm and that photons of wavelengths shorter than 191-207 nm have sufficient energy to exceed the threshold ionization energy of liquid water.
It is very well-known in the art that oxygen radicals, which are produced by numerous means, are highly reactive with a tremendous range of materials, causing degradation both in the presence and absence of water, depending upon the specific circumstances. Prevention of oxidation is a large and complex art in itself, with a long history.
One new development in the field is so-called immersion lithography as described in Switkes et al, J. Vac. Sci. Technol. B, 19 (6), 2353 6, November/December 2001; and, M. Switkes et al, “Resolution enhancement of 157-nm lithography by liquid immersion”, Proc. SPIE Vol. 4691, pp. 459465 (2002). In immersion lithography, the optical source, the target surface, or the entire lithographic apparatus is immersed in a highly transparent high refractive index liquid. Realization of the potential benefits of this technology is dependent upon identifying liquids with exceptionally high transparency in the VUV with excellent photochemical stability, as described, for example, in M. Switkes, R. R. Kunz, M. Rothschild, R. F. Sinta, P. M. Gallagher-Wetmore, and V. J. Krukonis, “Liquids for immersion lithography at short wavelengths”, Proc. SPIE Vol. 5040, 690-699 (2003).
Switkes et al, Microlithography World, May 2003, pp. 4ff, further demonstrate that light in a high refractive index medium can simulate the effects of much shorter wavelength light for photolithographic purposes. 193 nm light can be made to behave as though it has a wavelength of 136 nm and 157 nm light can be made to behave as though it has a wavelength of 115 nm by completely filling the gap between lens and wafer with a liquid fluorocarbon called an immersion fluid. According to Switkes et al, “Liquids for immersion lithography at short wavelengths,” op. cit., an immersion liquid layer must be at least 1 mm thick for mechanical reasons and at least 95% transparent for good optical performance. A reasonable estimate of the needed absorbance for an immersion liquid is A/cm=0.22, as determined from the equationA/cm≦[log10(To/T)]/h=[log10(100%/95%)]/0.1=0.22
Where To=light intensity in the absence of immersion fluid                T=light intensity with immersion fluid present        h=distance from lens to resist in centimetersIn general, of course, the more transparent the better.        
All known organic materials absorb to some extent at 157 nm. B. A. Lombos et al Chemical Physics Letters, 1, 42 (1967) discloses that short chain hydrocarbons H(CH2)nH with n<2; and G. Belanger et al Chemical Physics Letters, 3(8), 649(1969), and K. Seki, et al, Phys. Scripta, 41, 167(1990) disclose that short chain fluorocarbons F(CF2)nF with n≦6 to 10 are likely to be relatively transparent compared to longer chain homologues. French et al, WO 01/37044 and WO 01/85811 disclose polymers having alternating fluorocarbon segments and hydrocarbon segments which are much more transparent materials at 157 nm than either (CH2)n or (CF2)n as chain lengths grow longer.
There is a need for lower molecular weight organic compositions, particularly liquids, which may be employed, for example, as solvents for the polymer during spin coating, as plasticizers for polymeric films, or in an adhesive formulation. Alternatively, an organic fluid or gel may be employed as an immersion medium in immersion photolithography, as disclosed for example by Switkes and Rothschild, op. cit., in which a liquid medium is used between the projection lens of the optical stepper and the photoresist coated substrate (typically a silicon wafer) which will receive and detect the photolithographic image.
Hydrofluorocarbons having the general formula CnF2n+2−xHx are well known in the art, and are readily prepared by known methods. One such method is the addition of hydrogen across the double bond of a fluoroolefin or a hydrofluorocarbon olefin using nickel or palladium as a catalyst as described in M. Hudlicky, Chemistry of Organic Fluorine Compounds, 2nd Edition, John Wiley and Sons, New York, 1976 pages 174 and 175. In the alternative, said hydrofluorocarbons may be prepared by the reduction of Br, Cl, and I atoms in fluorocarbons or hydrofluorocarbons to H with inorganic reducing agents such as LiAlH4 or Zn as described in Hudlicky, op. cit., page 182 or alternatively on page 189. In yet another method, said hydrofluorocarbons may be prepared using organic reducing agents such as tributyitinhydride as described in Hudlicky, op. cit., page 197.
F[CF(CF3)CF2O]nCF2CF3, is known in the art, as described in, Modern Fluoropolymers, J. Scheirs, editior Chapter 24, “Perfluoropolyethers (Synthesis, Characterization, and Applications)” John Wiley & Sons, New York, 1997. F[CF(CF3)CF2O]nCFHCF3 where n=1 to 5 comes from an intermediate in the synthesis of F[CF(CF3)CF2O]nCF2CF3 in which a —COOH end group has been thermolysed to hydrogen rather than converted to —F with fluorine gas. X—Rfa[ORfb]nORfcY wherein X and Y can be hydrogen or fluorine and Rfa, Rfb, and Rfc are 1 to 3 carbon fluorocarbon radicals, linear or branched is a known variation of F[CF(CF3)CF2O]nCF2CF3, also described in Modern Fluorpolymers, op. cit. HCF2(OCF2)n(OCF2CF2)mOCF2H where n+m=1 to 8 is a variation of the synthesis of said X—Rfa[ORfb]nORfcY in which the end groups are not fluorinated but rather diverted to other chemistry as described in Modern Fluoropolymers, op. cit., on p. 441 happens to show end groups being reduced to CH2OH rather than converted to H). Not all of the variations implied by the generic formulas may be known or easily made: for example Class ii where one has H[CF(CF3)CF2O]nCF2H.
CF3CH2CF2CH3 is known to be synthesized by reacting CCl4 and CH2═CClCH3 to give CCl3CH2CCl2CH3, and then replacement of the chlorines by treatment in hydrofluoric acid. See R. Bertocchio, A. Lantz, L. Wedlinger, Chem. Abstracts 127:161495.
Polyperfluoroformaldehyde has not been reported except U.S. Pat. No. 6,193,952 described using [CF3O(CF2O)nCF3] (n=1-4) as a gas emulsion agent for ultrasound contrast enhancement. However, that patent does not teach how to prepare these compounds. Perfluoroether, CF3OCF2OCF2CF2OCF2OCF3, has been reported by direct fluorination from the corresponding hydrocarbon ether (U.S. Pat. No. 5,322,904). Electrochemical dimerization of fluoroacid, CF3OCF2CF2CO2H, has been reported (Novosti Elektrokhim. Org. Soedin., Tezisy Dokl. Vses. Soveshch. Elektrokhim. Org. Soedin., 8th Meeting Date 1973, 31-2. CA 82:36563).